The subject of the invention is a process for the continuous preparation of an aqueous alkaline suspension of low-grit, crystallized zeolitic sodium aluminosilicate of the smallest particle size by crystallization of an aqueous alkaline suspension of x-ray-amorphous sodium aluminosilicate.
X-ray-amorphous sodium aluminosilicates normally are prepared by the reaction of an aqueous sodium aluminate solution with an aqueous sodium silicate solution in the presence of excess sodium hydroxide solution at temperatures above room temperature. Generally the batch and concentration ranges of the reaction partners in the conventional technology correspond to molar ratios of: EQU 1.5 to 5 Na.sub.2 O: 1 Al.sub.2 O.sub.3 : 1 to 4 SiO.sub.2 : 40 to 400 H.sub.2 O.
Such batch ratios always produce a suspension of an x-ray-amorphous hydrous sodium aluminosilicate with a large excess of sodium hydroxide solution. The amorphous, hydrous solid has a chemical composition corresponding to the molar ratios of 1 to 5 Na.sub.2 O: 1 Al.sub.2 O.sub.3 : 1.8 to 4 SiO.sub.2. After the separation of the mother liquor and rinsing out the excess alkali, x-ray-amorphous products with a chemical composition corresponding to molar ratios of EQU 0.9 to 1.1 Na.sub.2 O: 1 Al.sub.2 O.sub.3 : 1.8 to 4 SiO.sub.2
with a moisture content depending on the degree of drying, can be isolated. The silicate content of the amorphous product is largely determined by the molar ratio of SiO.sub.2 : Al.sub.2 O.sub.3 in the reaction batch.
However, instead of such amorphous sodium aluminosilicates, their crystalline and preferably zeolitic products, subsequently obtained, are used for most technical applications. The so-called zeolites form a mineral class of alkali metal aluminosilicates with water of crystallization and with defined pore and spatial structure of their aluminosilicate lattice. "Molecular sieves" are those zeolites of these lattice characteristics that are used, particularly for the separation of substances. Synthetic zeolites are acquiring increasingly technical significance and are utilized as cation exchangers especially for the softening of water, as catalysts in chemical processes, as drying, separation or adsorption agents for solvents and gases, and as heterogeneous inorganic builders in detergents and cleaning agents. Depending on the purpose for which they are to be used, different types, and degrees of dryness and purity are needed. Usually, such molecular sieves first are prepared in their sodium form and then converted into other forms by cation exchange. Molecular sieve NaA is of special importance for most of the mentioned applications. The chemical composition of this type corresponds to the empirical formula: EQU 0.9 to 1.1 Na.sub.2 O: 1 Al.sub.2 O.sub.3 : 1.8 to 2.5 SiO.sub.2 : 0 to 6 H.sub.2 O.
The characteristic x-ray diffraction diagram of this zeolite is described, for example, in the U.S. Pat. No. 2,882,243.
A sodium aluminosilicate of small particle size with a grain size distribution within limits as narrow as possible and a mean grain size below 10 .mu.m is preferred for most technical applications. For the use in detergents and cleaning agents, there is the additional requirement that the proportion of particles having a particle size above 25 .mu.m shall not amount to more than 0.2 percent by weight and that the cation exchange capability shall be as high as possible. The production of particles exceeding 25 .mu.m is determined by wet-screening according to MOCKER (DIN 53 5801) and is called "grit" in the following text. The parameter used to determine the cation exchange capability is the calcium-binding capacity of 1 gm of crystalline sodium aluminosilicate (active substance) per liter of 30.degree. d (German hardness) initial hardness after a reaction time of 10 minutes at room temperature.
The conversion of amorphous sodium aluminosilicate into zeolitic forms is a crystallization process that depends on many parameters and proceeds at a faster rate with increasing temperature. Technically preferred is the crystallization at normal pressure for the preparation of molecular sieve NaA, particularly at temperatures above 70.degree. C. Highly elevated temperatures, for example, far above 100.degree. C. (in autoclaves), generally encourage the formation of other crystalline types of sodium aluminosilicates with strongly reduced cation exchange capability. Depending on the molar ratios in the batch and the temperature, this crystallization step requires times from a few minutes to several days. The crystallization time needed for the preparation of a highly crystalline product of the NaA type in a high volume/time yield, at particularly preferred molar ratios in the batch in the range of 1.5 to 5 Na.sub.2 O: 1 Al.sub.2 O.sub.3 : 1 to 2.5 SiO.sub.2 : 60 to 140 H.sub.2 O and crystallization temperatures in the range from 70.degree. to 100.degree. C., generally is 15 to 60 minutes. An extended crystallization time is needed to achieve products that are especially low in grit.
Technically amorphous sodium aluminosilicate can be prepared discontinuously as well as continuously. The conversion of this amorphous sodium aluminosilicate into small-sized zeolitic products with a grit content below 0.2 percent by weight, however, can be produced according to the prior art on a technical scale only by a discontinuous method.